Inductive loop vehicle presence detector

ABSTRACT

A vehicle detecting system includes a loop oscillator, a reference oscillator, a mixer for generating a difference frequency signal, and a low-pass filter for passing a normal, low frequency signal from the mixer. When a vehicle moves over the loop, the frequency of the loop oscillator increases, and the difference frequency likewise increases and is rejected by the low-pass filter to provide an output signal. To stabilize the system, a switching circuit is responsive to the output signal and is operable to decrease the reference oscillator frequency somewhat to provide hysteresis for partially self-sustaining the output signal when the low-pass filter rejects the difference frequency.

United States Patent 2,679,005 /1954 Batailleetal. 3,205,352 9/1965Prucha Primary Examiner-Donald J. Yusko Anorneys-Francis L. Masselle,William Grobman and .Edward A. Robinson ABSTRACT: A vehicle detectingsystem includes a loop oscillator, a reference oscillator, a mixer forgenerating a difference frequency signal, and a low-pass filter forpassing a normal, low frequency signal from the mixer. When a vehiclemoves over the loop, the frequency of the loop oscillator increases, andthe difference frequency likewise increases and is rejected by thelow-pass filter to provide an output signal. To stabilize the system, aswitching circuit is responsive to the output signal and is operable todecrease the reference oscillator frequency somewhat to providehysteresis for partially selfsustaining the output signal when thelow-pass filter rejects the [72] lnventor Martin John Prucha MountainView, Calif. [21] Appl. No. 653,925 [22] Filed July 17, 1967 PatentedApr. 27, 1971 [73] Assignee Singer-General Precision, Inc.

Binghamton, N.Y.

[54] INDUCTIVE LOOP VEHICLE PRESENCE DETECTOR 8 Claims, 5 Drawing Figs.

[52] US. Cl 340/38, 331/43, 331/65, 340/258 [51] Int. Cl. G08b 13/00,608g 1/00 Field ofSearch 340/31, 38, 51, 258; 331/37, 38, 42, 43, 64,65, 117; 324/3 [56] References Cited UNITED STATES PATENTS 2,606,2858/1952 Bataille et a1. 250/20 -35 53 32 E ll\ 3P 7 29 so 15,36 42 l i assee- 5:

32' 35' I I E v PATENT EUAPR2Tl97l 7 3576.525 SHEET 1 UF v 1 5&8

INVE/VTUI? MARTIN JOHN PRUCHA w o 0d $55 I. W 5 5mm mmfi 30 Q 9PATEN-TEBAPRZYIQ?! 33576525 SHEET 2 BF 2 CONST-K M-DERIVED CONST-KSECX'ION sEcT mN ssc nom OUTPUT 03. 75 FT 7'6 A A A r I V FIG. 3a

, FREQ-M OUTPUT n. 5. 75' 7a 79 r A PM! A FIG. 3b

FREQ-h OUTPUT D.B.

FIG. 3C

FREQ

INDUCTIVE LOOP VEHICLE PRESENCE DETECTOR This invention relates toapparatus for sensing the presence of a motor vehicle, a railroadvehicle or the like, and, more particularly, this invention relates tosuch apparatus including an oscillator, the output frequency of which isaffected by the introduction of the metallic mass of the vehicle into amagnetic field established by an inductive loop that is in theelectrical circuit of the oscillator. This patent application is adivision of a copending prior application, Ser. No. 172,620, entitled,Method for Tuning Inductive-Loop Vehicle Detectors," filed by thisinventor on Feb. 12, 1962, now U.S. Pat. No. 3,364,465.

Inductive-loop presence detecting systems have been disclosed andclaimed by a U.S. Pat. No. 3,164,802, entitled, Inductive-Loop VehiclePresence Detector, granted to Robert A. Kleist and John Scarbrough onJan. 5, i965; and by another U.S. Pat. No. 3,205,352, entitled, PresenceDetector, issued on Sept. 7, 1965, to the inventor of the instantapplication. Both of said patents are assigned to the same assignee asthe instant application.'These patents disclose arrangements for sensingvehicles wherein inductive loops are electrically connected intooscillator circuits such that the inductive value determines thefrequency of oscillation. The frequency of each loop oscillator may becompared with the frequency of a corresponding reference oscillator toobtain a difference or beat frequency. If the frequency of the looposcillator varies slightly with respect to that of the referenceoscillator, the difference or beat frequency will vary considerably andmay be utilized for generating an output signal indicative of thepresence of a vehicle within the field of the loop.

It is an object of this invention to provide an improved loop oscillatorarrangement for sensing the presence of a vehicle, and, moreparticularly, it is an object to provide such an arrangement wherein areference oscillator may be tuned to provide an output signal of a firstfrequency during times when no vehicle presence is sensed within thefield of the loop oscillator, and wherein the reference frequency may beshifted when a presence signal appears such that the system is partiallyselfsustaining and is stabilized.

Another object of this invention is to provide an improved presencedetector system using a filter for selectively passing the beatfrequency signals generated by a comparison of the signals of the looposcillator and the reference oscillator.

A further object isto provide an improved oscillator arrangement whereinthe loop oscillator and the reference oscillator have similar circuitssuch that oscillator drift due to temperature and environmental changeswill similarly affect both oscillators, and the difference or beatfrequency variation will be minimized.

Another object is to provide a stabilized loop oscillator wherein theinductive loop may be electrically isolated from ground referencepotential such that a variation in frequency due to changes in the straycapacitive effect of the loop will be minimized. Numerous other objectsand advantages will be apparent throughout the progress of thespecification which follows. The accompanying drawings illustrate acertain selected embodiment of the invention and the views therein areas follows:

FIG. 1 is a'circuit diagram of the presence detector system of thisinvention; v

FIG. 2 is a circuit diagram of a low-pass filter which was shown as ablock in FIG. I; and

FIGS. 3a, 3b, and 3c are graphical representations of the responsecharacteristics of the low-pass filter illustrated in FIG. 2.

Briefly stated, according to a preferred embodiment of this invention,the vehicle detector system includes two similar oscillator circuits l1and 12. The frequency of oscillation of the circuit 11 is determined bya tuned circuit including a capacitor 13 and an inductive loop M whichmay be a single turn or electrical conductor embedded in the paving of atraffic lane of a street or placed under or between the rails of arailroad track. A magnetic field is established by the loop l4,

and the inductive value of 'the loop will be decreased when the metallicmass of a vehicle enters the field of the loop. The loop oscillator 11generates a signal of a frequency determined by the inductive value ofthe loop 14, which signal is combined with a signal from the referenceoscillator 12 by a difference frequency detector including a transistor15. A low-pass filter 16 will pass the difference signal when no vehicleis in the field of the loop but will reject-the difference signal, whichincreases in frequency when a vehicle enters the field of the loop. Thedifference signal is amplified and rectified by the circuits l7 and 18to provide a direct-current output signal. A transistor switch 20becomes conductive when the signal from the amplifier 17 falls off and acapacitor 21 is effectively shunted across a resonant circuit includingan inductance 22 and a capacitor 23 whereby the frequency of thereference oscillator is decreased somewhat. The decrease in frequency ofthe reference oscillator further increases the difference frequencygenerated by the transistor 15 and causes a hysteresis effect such thatthe presence detector system becomes partially self-sustaining and isstabilized.

The loop oscillator 11 includes two transistors 28 and 29. The firsttransistor 28 is essentially a grounded-base amplifier, and the secondtransistor 29 is essentially an emitter follower. The base electrode ofthe transistor 28 is coupled to a biasing network including resistors 30and 31. The resistor 31 provides a negative feedback path forstabilizing the circuit. A resistor 32 and a capacitor 35 provide an RCnetwork for coupling the negative potential, -15, of a power supply toboth transistors 28 and 29 while providing a shunt path to ground foralternating currents. A resistor 34 constitutes a load impedance for thetransistor 28, and the base electrode of the transistor 29 is directlyconnected thereto. A resistor 36 constitutes a load impedance for thetransistor 29 and provides a positive feedback path to sustainoscillation between the transistors 28 and 29. The frequency of theoscillations is determined by a resonant circuit including the inductiveloop 14 and the capacitor 13, which is coupled to the positive feedbackpath via a transformer 37. The resonant circuit 13-14 and transformer 37provide a shunt path to ground which is of relatively low impedance forall frequencies except the resonant frequency. Therefore, the positivefeedback through the resistor 36 will be of the frequency correspondingto the highimpedance shunt path of the tuned circuit 13-14. The resonantfrequency of the circuit 13-14 is dependent only upon the inductive andcapacitive values of the elements therein. The inductive value of a loophas been found to be essentially constant regardless of changes inenvironmental conditions. To stabilize and render the oscillator circuitinsensitive to environmental changes and particularly to changes inground moisture, the circuit is arranged such that the value of the loopinductance and the value of the capacitor 13 will predominate, and theeffect of the other factors, such as stray capacitance will beminimized. The means for minimizing the effect of stray capacitance ofthe loop will be discussed subsequently. Changes in ground conductancewill not affect the inductance value of the loop, although such changesmay cause variation in the Q value. However, the oscillator 11 isinsensitive to the Q of the resonant circuit 13-14, and therefore,variation in the Q will not affect the oscillator frequency.

The reference oscillator 12 is a nearly identical circuit to the looposcillator 11, and the circuit components thereof shown in FIG. Iare-identified by reference numerals of the circuit 11. The circuit 12differs from the circuit 11 in that the inductance winding 22 and thecapacitor 23 constitute the tuned circuit corresponding to the loop ldandthe capacitor 13 of the oscillator 11, and the tuned circuit 22-23 isdirectly connected to the emitter electrode "of the transistor 28',whereas a transformer coupling 37 is employed in the oscillator 11. Aswill be described subsequently, additional capacitors 38 and 39 may beconnected in parallel with the capacitor 23 by jumper connections 40 and51.

The difference or beat-frequency generating circuit includes thegrounded emitter transistor 15 having one of the oscillators coupled tothe base electrode thereof via a resistor 42 and having the otheroscillator coupled to the collector electrode via another resistor 43.Since the base electrode of the transistor is not coupled to a directpotential source, this transistor conducts during alternate half-cyclesof the input wave from the oscillator 11, and therefore, the operationis class B such that both radio frequencies are mixed and produce anenvelope wave which is the difference frequency. Both RF signalstogether with the audio difference signal will appear at the collectorelectrode of the transistor 15. An RF choke coil 45 attenuates the RFoscillator signals and passes the envelope or beat frequency, which isequal to the difference between the frequencies of the two oscillators.The difference frequency is normally such as to pass the lowpass filter16, and will appear at a point 46 at the output of the amplifier 17. Theaudio frequency is rectified by the circuit 18 to provide a directcurrent level at an output terminal 47. The circuit 17 may be aconventional audio amplifier, and the circuit 18 may be a conventionalbridge rectifier.

The audio wave normally appearing at the point 46 is passed by an RCnetwork including a capacitor 48 and a resistor 49 to a voltage-doublerrectifying circuit including two diodes 50 and 51. A positive potentialwill appear as a charge on a capacitor 52 when a wave appears at thepoint 46. A transistor 53 is normally conductive, the base electrodethereof being coupled to ground reference potential by a resistor 54,and the emitter electrode being coupled to the positive charge of thecapacitor 52 by a potential dividing network, including resistors 56 and57. When the low-pass filter blocks signals therethrough, no signalswill appear at the point 46, and the capacitor 52 will becomedischarged, eliminating the positive bias and causing the transistor 53to be rendered nonconductive by the negative bias of the referencepotential, E.

The transistor switch includes a pair of transistors and 26 which arenormally nonconductive, whereby the capacitor 21 is isolated from theground reference potential. Both the base electrodes of the transistor25 and 26 are nonnally biased positively through conduction of thetransistor 53. However, during times when the difierence signal isrejected by the lowpass filter 16, the transistor 53 becomesnonconductive, whereupon the base electrodes of the transistors 25 and26 are biased negatively via a resistor 59, and both transistors will berendered conductive.

The capacitor 21 couples the alternating signal from the referenceoscillator 12 to a point 61 which is normally isolated from ground,since the transistors 25 and 26 are nonconductive. It may be noted thatthe transistors 25 and 26 are connected essentially in parallel, butwith one transistor inverted with respect to the other. The two baseelectrodes are connected together and to a common biasing controltransistor 53, but the collector electrode of one transistor isconnected to the emitter electrode of the other transistor, in eachcase, and the two emitter-collector electrode combinations are coupledbetween the point 61 and the ground reference potential. When thetransistors 25 and 26 are biased into conduction, the alternating wavefrom the oscillator 12 is passed by a capacitor 21 through thetransistors 25 and 26 to the ground reference potential. Alternatehalf-cycles of the wave are of opposite polarity, and a singletransistor connected between the point 61 and ground would conductnonsymmetrically, since the saturation impedance of a transistor isrelated to the amplification characteristic, B, and the [3characteristics of transistors are considerably different when theemitter and collector electrodes are interchanged. During the positivelobes or half-cycles of the alternating wave, the transistor 26 willconduct with substantially no impedance while the transistor 25, beingreversed, will conduct but will offer some degree of impedance tocurrent flow. During the negative halfcycles of the wave, the transistor25 will be fully conductive while the transistor 26 will offer someimpedance. Thus, it may be appreciated that during the positiveexcursions of the alternating wave, the transistor 26 will provide aprimary path to ground, and during the negative excursions, thetransistor 25 will provide a primary conduction path. Therefore, thecombination of the two transistors, inverted with respect to each other,will provide a good conduction path for the alternating wave, regardlessof the instantaneous polarity thereof. This inverted transistorarrangement 25-26 constitutes an effective switching means foralternating currents.

Alternatively, a symmetrical transistor could be used in place of thetwo transistors 25 and 26. A symmetrical transistor is commerciallyavailable and includes a base electrode of one type of semiconductorbetween two symmetrical electrodes of the opposite type ofsemiconductor. With this type of transistor, the base electrode and thecollector electrode are interchangeable and the amplification factor, B,is the same regardless of which of the side electrodes is used as anemitter and which as a collector. However, the cost of aspecial-purpose, symmetrical transistor is greater than the cost of twoordinary transistors connected as indicated by transistors 25 and 26 inFIG. 1.

As described heretofore, the tuned circuit, including the capacitor 13and the loop 14, is coupled into the loop oscillator 11 by thetransformer 37, which provides a ground connection required by theoscillator circuit 11 while isolating the loop 14 from ground. The loop14 is an insulated copper wire laid on or embedded in the ground, and acertain amount of stray capacitance exists between the wire and thesurrounding ground. In FIG. 1, capacitors 65 and 66 are not elementsconnected into the circuit, but exist because the loop 14 is a metallicconductor physically lying on or embedded within the surface or streetpaving, etc. The existence of the stray capacitance 6566 is notdetrimental in itself because the capacitor 13 may be adjusted tocompensate for the capacitance 65-66 and to cause the circuit 11 tooscillate at the proper frequency. The capacitors 65-66 are shownconnected to opposite sides of the loop 14, which is a goodapproximation to the actual distributed capacitance. Since there is noactual ground connection, the loop is balanced and, from an electricalstandpoint, the capacitance 65 on one side of the loop is in series withthe capacitance 66 on the other side; the two capacitances in series areshunted across the capacitor 13. While the existence of the capacitance65 and 66 is not detrimental, any changes or variance in thiscapacitance may be very detrimental. Such changes may come about becauseof changing environmental conditions and particularly because of changesin the moisture content of the ground wherein the loop 14 is installed.Thus, the capacitance 65-66 may substantially increase during a rainstorm and may cause variation in the tuning of the loop oscillator. Tominimize the variation of the stray capacitance, the effect of thecapacitance itself should be minimized. it is desirable that no actualground connection be made to the loop whereby the two capacitances 65and 66 exist in series from oneside to the other of the loop. If oneside of the loop were grounded, the effect of assumed stray capacitanceswould be coupled across the loop and would have a more detrimentaleffect on the system. It is further desirable that the loop be of aminimum number of turns, and actual loop installations have provensatisfactory wherein the loop is but a single turn. in this case, thestray capacitance 65-66 is minimized and the effect of changes thereinis not detrimental to the operation of the loop oscillator 11. indeed,it has been found that the transformer 37 may have a turns ratio on theorder of 3:1 to permit the use of a single-tum loop to minimize theeffect of the stray capacitance. In the event that a multiple-turn loopis found to be necessary, the several conductors required may be encasedwithin a single, waterproof cable covering or sheath. This arrangementwill minimize changes in the stray capacitance between the turns of theloop, and the primary change in stray capacitance will be between theseveral conductors considered as a whole and the surrounding ground.

The low-pass filter 16 includes three inductive windings 69, 70, and 71and three capacitors 72, 73, and 74, as shown in FIG. 2. A portion ofthe inductance 69 together with the capacitor 72 constitutes one-half ofa constant-K filter section,

and similarly, a portion of the inductance 70 together with thecapacitor 73 constitutes another half of a constant-K filter section.The remainder of the inductance 69, the remainder of the inductance 70,the inductance 71, and the capacitance 74 constitute an M-derived filtersection. FIG. 3a shows the response characteristic for a constant-K,low-pass filter. This characteristic provides a pass band 75 for thelower frequency signals and a rejection band 76 for higher frequencysignals. Between the pass band 75 and the rejection band 76 lies a band77 wherein the signal response in decibels decreases linearly as thefrequency increases. Since the response characteristic of an ideallow-pass filter would have a low frequency pass band and a sharp cutofffollowed by a high frequency rejection band, the constant-K filter failsas an ideal filter because the cutoff is not sharp but is gradual in theregion 77.

FIG. 3b shows the response characteristic of an M-derived filter. Thisfilter section provides a low frequency pass band 75, a narrow rejectionband 78, and a high frequency pass band 79. The M-derived filter may besharply tuned such that the rejection band is narrow and the responsecuts off sharply from the pass band into the rejection band. The sharpcutoff of the M-derived section is combined with the low-pass andhighrejection bands of the constant-K section to provide a combinedresponse curve as shown in FIG. 3c. The combined characteristic includesa low frequency pass band 75" corresponding to both pass bands 75 and75' of the constant-K and M-derived sections. As the frequency isincreased beyond the pass band, the response cuts off sharply due to thesharp cutoff characteristic of the M-derived section. A minimum responseband or notch" 80 also results from use of the M- derived section. Asthe frequency increases from the minimum response point 80, the outputresponse will increase slightly but not sufficiently to cause switchingin the presence detector circuits. It may be appreciated that thecombination of the constant-K sections with the M-derived sectionsprovides a low-pass filter having a uniform pass band for lowfrequencies and a sharp cutoff to the minimum response point 80.

As shown in FIG. 3c, the switching level 81 may be established as 6 db.below the normal response of the pass band 75", and is therefore set atapproximately 0.5 of the full response of the filter. The minimum point80 would have a zero response if the M-derived filter section couldinclude ideal circuit elements. However, with actual circuit elementsavailable, the minimum point 80 may be 30 db. down from the pass bandlevel 75". As the frequency increases beyond the minimum point 80, theresponse level rises somewhat to a value of db., which is obviously wellbelow the 6 db. switching level 81.

In an exemplary embodiment of this invention, a presence detector systemhas been constructed wherein the loop oscillator is tuned to 90kilocycles with no vehicle present in the field of the loop. Thereference oscillator 12 is normally tuned to a frequency of 87.25kilocycles, and, under these conditions, a beat frequency of 2,750cycles (90kc.87.25 kc.) is generated by the difference frequencydetector 15, and is passed by the low-pass filter l6 and the poweramplifier 17. When the metallic mass of a vehicle enters the fieldof theloop 14, the frequency of the loop oscillator is caused to shift to avalue in excess of 92.5 kilocycles, whereby a beat frequency greaterthan 5,250 cycles will be generated by the differencefrequency generator15. The frequency of 5,250 cycles corresponds to the 6 db. level of thefilter, indicated by a point 82 on the characteristic curve of FIG. Be.At this level, the transistor switch 20 will effectively close andcouple the capacitor 21 in parallel with the capacitor 23 of thereference oscillator 12. The reference oscillator 12 is thereby shiftedin frequency from 87.25 kc. to 85.25 kc., and the beat frequency willincrease to 7,250 cycles, which is well in excess of the switching point82. Therefore, when the threshold value is reached, a positive switchingaction will occur, and the system will be partially self-sustaining tohold the system and prevent an immediate reverse switching. Similarly,when the metallic mass is removed from the loop, the loop oscillatorwill again return to a 90 kc. output signal, whereupon the beatfrequency will be reduced to 4,750 cycles. Since the 4,750 cycles isdefinitely below the switching frequency of 5,250 cycles, the transistorswitch 20 will open and decouple the capacitor 21 from the resonant ortank circuit of the oscillator 12, whereby the output from the referenceoscillator 12 will return to the normal value of 87.25 kc. It may beappreciated that a hysteresis effect is introduced by causing thereference frequency to shift slightly when switching action occurs, toprevent unstable or relay chatter conditions from occurring.

The reference oscillator may be assembled and pretuned in manufacture.However, the loop oscillator may not be completely assembled and may notbe tuned at the factory because the inductive loop 14 must be installedat a field location and thence connected into the oscillator circuit.Obviously, each installation may be somewhat different, and theinductive value of the loop cannot be predetermined to permit apretuning of the oscillator 11. Therefore, the capacitor 13 may beadjusted as indicated in FIG. 1 to facilitate a final field tuning ofthe loop oscillator 111 after the installation is completed. To furtherfacilitate the tuning of the loop oscillator, the capacitor 38 may becoupled into the resonant circuit of the oscillator 12 by the jumperconnection 40, and the reference oscillator will be caused to shift to alower frequency output. In the exemplary embodiment discussed above, thecapacitor 38 was of a value of 3,000 mmf., and caused the oscillator toshift to a frequency of 84.25 kc. Since the lowpass filter 16 provides aminimum point or notch frequency equal to 5,750 cycles. The looposcillator may then be tuned to a minimum response from the low-passfilter 16 and amplifier 17. This minimum response characteristic,corresponding to the notch point 80, FIG. 30, is easily identifiablewith minimum apparatus. For example, a simple voltmeter coupled to theoutput terminal 47 c). provide the necessary indication of minimumresponse as the capacitor 13 is adjusted under field conditions.Alternatively, a pair of headphones may be coupled to the output of theamplifier 17 to provide an audible indication of minimum response as thecapacitor I3 is varied. With the reference frequency shifted to 84.25kc. by the capacitor 38, the loop oscillator frequency must be tuned tokc. to achieve the minimum response at the 5,750-cycle point 80 of theresponse curve (FIG. 30). Thus, it may be appreciated that the referenceoscillator, which was pretuned at the factory, and the notchcharacteristic 80 of the low-pass filter may be used in the field fortuning the loop oscillator 11, using a minimum of field equipment.

The sensitivity of the vehicle presence detector system may be increasedsomewhat by adding further capacitance to the resonant circuit of thereference oscillator 12. FIG. 1 shows the additional capacitor 39 whichmay be coupled in parallel with the principle capacitor 23 by means ofthe jumper connection 41. The addition of the capacitor 39 to theresonant circuit causes the frequency of the reference oscillator 12 todecrease somewhat, thereby increasing the beat or difference frequencygenerated by the transistor 15. With an increased beat frequency, thethreshold level of the low-pass filter 16 is more closely approached,and, therefore, the loop oscillator 11 will be more critical inoperation. The presence detector system with the additional capacitance39 coupled into the reference oscillator 12 will be more sensitive tovehicles which may approach the field of the loop 14. The amount ofcapacitance 39 which may be added for the purpose of increasing thesensitivity of the system must be limited by considerations of thestability of the system. Obviously, if the threshold level of thelow-pass filter were too closely approached, spurious or false switchingactions may result from slight deviations from the normal frequency byeither of the oscillators 11 or 12. In the exemplary embodimentdescribed above, the vehicle presence sensing apparatus included severalsmall capacitors 39 which could be introduced into the circuit by jumperconnections, as indicated at 41, such that the sensitivity of the systemcould be adjusted in the field, and

7 a fair compromise between good sensitivity and good stability may beobtained.

I claim:

1. Apparatus for sensing the presence of a vehicle, said apparatuscomprising a first oscillator including an inductive loop, said firstoscillator being operable to generate a signal which will increase infrequency when a vehicle enters a field established by the loop, asecond oscillator for generating a reference signal of a frequencydiffering from the frequency of the loop oscillator. a beat frequencydetector coupled to both oscillators and operable to generate a signalhaving a frequency equal to the difference between the frequencies ofthe oscillator signals, a low-pass filter coupled to the beat frequencydetector, means coupled to the low-pass filter for generating a directvoltage corresponding with the amplitude of waves passed by the filter,and means controllably coupled to the reference oscillator and coupledto receive the direct voltage whereby the reference frequency of thesecond oscillator is shifted when the direct potential level varies.

2. Apparatus for sensing the presence of a vehicle, said apparatuscomprising a first oscillator including an inductive loop, said firstoscillator being operable to generate a signal which will increase infrequency when a vehicle enters a field established by the loop, asecond oscillator for generating a reference signal, the referencesignal having a normal frequency less than the frequency of the firstoscillator, a beat frequency detector coupled to both oscillators andoperable to generate a signal having a frequency equal to the differencebetween the frequencies of the oscillator signals, a low-pass filtercoupled to the beat frequency detector and operable to pass thedifference frequency signal when both oscillators are generating normalsignals, said low-pass filter being further operable to block thedifference frequency signal when a vehicle enters the loop and the firstoscillator increases in frequency, and a switching means coupled to thelow-pass filter and controllably coupled to the second oscillator forcausing the second oscillator to decrease in frequency when the low-passfilter blocks the flow of the difference frequency signal.

3. Apparatus for sensing the presence of a vehicle, said apparatuscomprising a first oscillator including an inductive loop, said firstoscillator being operable to generate a signal having a normal frequencyand being further operable to generate a signal of increased frequencywhen a vehicle enters a field established by the loop, a secondoscillator for generating a reference signal having a normal frequencyless than the frequency of the first oscillator, a beat frequencydetector coupled to both oscillators and operable to generate a signalhaving a frequency equal to the difference between the frequencies ofthe oscillator signals, a low-pass filter coupled to the beat frequencydetector and operable to pass the difference frequency signal when bothoscillators generate normal signals, said low-pass filter being furtheroperable to block the difference frequency signal when a vehicle entersthe loop and the first oscillator increases in frequency, said secondoscillator including a tuned circuit having a capacitor therein forestablishing the frequency of oscillation, a switching means coupled tothe low-pass filter, and a further capacitor coupled between theresonant circuit of the second oscillator and the switching means, saidswitching means being operable to coupie the second capacitor in shuntwith the first capacitor when the low-pass filter blocks the flow of thedifference frequency signal whereby the frequency of the. secondoscillator is decreased below the nonnal frequency thereof.

4. Apparatus in accordance with claim 3 wherein the switching meanscomprises two transistors each having an emitter electrode, a baseelectrode and a collector electrode, the emitter electrode of a first ofthe transistors being connected to the collector electrode of the secondtransistor and being further connected to the second capacitor, thecollector electrode of the first transistor and the emitter electrode ofthe second transistor being connected together and being connected to apoint of reference potential, the base electrode of both transistorsbein connected together and a biasin means responsrvely couple to thedifference frequency srgna passed by the low-pass filter, and coupled tothe base electrode of both transistors.

5. Apparatus in accordance with claim 3 wherein the lowpass filtercomprises a constant-K section and an M-derived section and wherein thefilter provides a characteristic including a low-pass band, a point ofminimum response and a high frequency rejection band, and wherein thesecond oscillator includes a capacitive means for shifting the signalfrequency thereof whereby the first oscillator may be tuned to a minimumresponse point of the filter characteristic.

6. Apparatus in accordance with claim 3 wherein the first oscillatorcomprises a resonant circuit including the loop and a tuning capacitorconnected across the loop, a coupling transformer having a windingcoupled across a loop and the tuning capacitor and having anotherwinding coupled to a reference potential whereby the resonant circuit ofthe loop is floating and ungrounded to minimize the effect of straycapacitance.

7. Apparatus for sensing the presence of a vehicle, said apparatuscomprising two oscillators tuned to different frequencies, a beatfrequency circuit coupled to receive signals from both oscillators andoperable to generate a signal of a frequency equal to the differencebetween the oscillator frequencies, a low-pass filter coupled to receivethe signal from the beat frequency circuit, an output circuit coupled tothe low-pass filter, each of the oscillators including a tuned circuitfor detennining the oscillator frequency, a first of the oscillatorshaving an inductive loop coupled thereto as a part of the tuned circuitand as a frequency determining element whereby a vehicle may cause avariation in the inductive value of the loop and cause a correspondingvariation in the frequency of the oscillator, a transistor switchresponsively coupled to the output circuit, said transistor switch beingcoupled to the tuned circuit of one of the oscillators and beingoperable to vary the frequency thereof in response to signal variationfrom the output circuit whereby the presence ofa vehicle will cause avariation in the signal of the output circuit to partially selfsustainthe output signal.

8. The apparatus in accordance with claim 7 comprising a capacitorcoupled to the tuned circuit of said one oscillator and to thetransistor switch, whereby the capacitor will become a part of the tunedcircuit and will shift the frequency of said one oscillator when avehicle moves over the field of the loop.

1. Apparatus for sensiNg the presence of a vehicle, said apparatuscomprising a first oscillator including an inductive loop, said firstoscillator being operable to generate a signal which will increase infrequency when a vehicle enters a field established by the loop, asecond oscillator for generating a reference signal of a frequencydiffering from the frequency of the loop oscillator, a beat frequencydetector coupled to both oscillators and operable to generate a signalhaving a frequency equal to the difference between the frequencies ofthe oscillator signals, a low-pass filter coupled to the beat frequencydetector, means coupled to the low-pass filter for generating a directvoltage corresponding with the amplitude of waves passed by the filter,and means controllably coupled to the reference oscillator and coupledto receive the direct voltage whereby the reference frequency of thesecond oscillator is shifted when the direct potential level varies. 2.Apparatus for sensing the presence of a vehicle, said apparatuscomprising a first oscillator including an inductive loop, said firstoscillator being operable to generate a signal which will increase infrequency when a vehicle enters a field established by the loop, asecond oscillator for generating a reference signal, the referencesignal having a normal frequency less than the frequency of the firstoscillator, a beat frequency detector coupled to both oscillators andoperable to generate a signal having a frequency equal to the differencebetween the frequencies of the oscillator signals, a low-pass filtercoupled to the beat frequency detector and operable to pass thedifference frequency signal when both oscillators are generating normalsignals, said low-pass filter being further operable to block thedifference frequency signal when a vehicle enters the loop and the firstoscillator increases in frequency, and a switching means coupled to thelow-pass filter and controllably coupled to the second oscillator forcausing the second oscillator to decrease in frequency when the low-passfilter blocks the flow of the difference frequency signal.
 3. Apparatusfor sensing the presence of a vehicle, said apparatus comprising a firstoscillator including an inductive loop, said first oscillator beingoperable to generate a signal having a normal frequency and beingfurther operable to generate a signal of increased frequency when avehicle enters a field established by the loop, a second oscillator forgenerating a reference signal having a normal frequency less than thefrequency of the first oscillator, a beat frequency detector coupled toboth oscillators and operable to generate a signal having a frequencyequal to the difference between the frequencies of the oscillatorsignals, a low-pass filter coupled to the beat frequency detector andoperable to pass the difference frequency signal when both oscillatorsgenerate normal signals, said low-pass filter being further operable toblock the difference frequency signal when a vehicle enters the loop andthe first oscillator increases in frequency, said second oscillatorincluding a tuned circuit having a capacitor therein for establishingthe frequency of oscillation, a switching means coupled to the low-passfilter, and a further capacitor coupled between the resonant circuit ofthe second oscillator and the switching means, said switching meansbeing operable to couple the second capacitor in shunt with the firstcapacitor when the low-pass filter blocks the flow of the differencefrequency signal whereby the frequency of the second oscillator isdecreased below the normal frequency thereof.
 4. Apparatus in accordancewith claim 3 wherein the switching means comprises two transistors eachhaving an emitter electrode, a base electrode and a collector electrode,the emitter electrode of a first of the transistors being connected tothe collector electrode of the second transistor and being furtherconnected to the second capacitor, the collector electrode of the firsttransistor anD the emitter electrode of the second transistor beingconnected together and being connected to a point of referencepotential, the base electrode of both transistors being connectedtogether and a biasing means responsively coupled to the differencefrequency signal passed by the low-pass filter, and coupled to the baseelectrode of both transistors.
 5. Apparatus in accordance with claim 3wherein the low-pass filter comprises a constant-K section and anM-derived section and wherein the filter provides a characteristicincluding a low-pass band, a point of minimum response and a highfrequency rejection band, and wherein the second oscillator includes acapacitive means for shifting the signal frequency thereof whereby thefirst oscillator may be tuned to a minimum response point of the filtercharacteristic.
 6. Apparatus in accordance with claim 3 wherein thefirst oscillator comprises a resonant circuit including the loop and atuning capacitor connected across the loop, a coupling transformerhaving a winding coupled across a loop and the tuning capacitor andhaving another winding coupled to a reference potential whereby theresonant circuit of the loop is floating and ungrounded to minimize theeffect of stray capacitance.
 7. Apparatus for sensing the presence of avehicle, said apparatus comprising two oscillators tuned to differentfrequencies, a beat frequency circuit coupled to receive signals fromboth oscillators and operable to generate a signal of a frequency equalto the difference between the oscillator frequencies, a low-pass filtercoupled to receive the signal from the beat frequency circuit, an outputcircuit coupled to the low-pass filter, each of the oscillatorsincluding a tuned circuit for determining the oscillator frequency, afirst of the oscillators having an inductive loop coupled thereto as apart of the tuned circuit and as a frequency determining element wherebya vehicle may cause a variation in the inductive value of the loop andcause a corresponding variation in the frequency of the oscillator, atransistor switch responsively coupled to the output circuit, saidtransistor switch being coupled to the tuned circuit of one of theoscillators and being operable to vary the frequency thereof in responseto signal variation from the output circuit whereby the presence of avehicle will cause a variation in the signal of the output circuit topartially self-sustain the output signal.
 8. The apparatus in accordancewith claim 7 comprising a capacitor coupled to the tuned circuit of saidone oscillator and to the transistor switch, whereby the capacitor willbecome a part of the tuned circuit and will shift the frequency of saidone oscillator when a vehicle moves over the field of the loop.